1. FIELD OF THE INVENTION
The present invention relates to a process for producing water absorbent resin. More specifically, it relates to a process for producing water absorbent resin having high water absorbency under loading and high safety.
2. DESCRIPTION OF THE PRIOR ART
Water absorbent resins have been extensively used for hygienic materials such as paper diapers, sanitary materials, etc. as a material which can replace pulp or water absorbent paper, based upon its marvelous water absorbency, as well as for applications as water retaining material, water sealing material, etc. and food processing applications making use of their characteristics.
For water absorbent resins applied for hygienic materials, neutralized salts of polyacrylic acid or polymethacryl acid are generally used from the viewpoint of composition. However, when they are used for paper diapers, they should exhibit not only simply high water absorbency under a pressure-free state (absorption rate of urine at no load), but also high water absorbency under loading (absorption rate with load). In order to increase the water absorbency under loading, the crosslinking degree must be increased because the water absorbent resin must absorb urine and other liquids while overcoming the load. However, in the conventional method, if the dosage of copolymerizable crosslinking agent is increased for uniform crosslinking, the water absorbency under loading is increased by the increase of crosslinking degree but has a defect of decreasing the water absorbency under a pressure-free state. This decreasing of the water absorbency under pressure-free state restrains the improvement of the water absorbency under loading.
To solve this problem, methods have been proposed for lowering the crosslinking degree of the inner part of the water absorbent resin in order to maintain the water absorbency under a pressure-free state, and crosslinking the vicinity of the surface of a water absorbent resin particle with a crosslinking agent having at least 2 functional groups which react with the carboxylic acid group and/or carboxylic acid salt group. That is, surface crosslinking methods have been proposed. In this event, for the crosslinking agent, epoxy compounds represented by ethylene glycol diglycidyl ether, etc. (e.g. Japanese Patent Application Laid Open No. 57-44627), polyhydric alcohol represented by glycerin (e.g. Japanese Patent Application Laid Open No. 58-180223), polyvalent amine compounds, polyaziridine compounds, or polyvalent isocyanate compounds (e.g. Japanese Patent Application Laid Open No. 59-189103), polyvalent epoxy compounds having an amino group (e.g. Japanese Patent Application Laid Open No. 63-195205), and reactants of epihalohydrin with a low-molecular primary amine such as ammonia or ethylene diamine (e.g. Japanese Patent Application Laid Open No. 2-248404) are known.
However, because these crosslinking agents have a comparatively low molecular weight, the crosslinking agent penetrates comparatively deeply into the inside of the particles when surface crosslinking takes place. As a result, the inside of the particles is crosslinked, though it is not as much as that observed in uniform-crosslinking, and the water absorbency under pressure-free state decreases.
When polyhydric alcohol or polyvalent amine is used for the crosslinking agent, heating exceeding 180.degree. C. is generally required to have the crosslinking reaction take place, and treatment at such a high temperature brings about thermal crosslinking or thermal deterioration of the water absorbent resin itself. This results not only in difficulty of control over the crosslinking degree but also decreasing of the water absorbency under a pressure-free state as well as water absorbency under loading.
In addition, the crosslinking agents such as epoxy compounds represented by ethylene glycol diglycidyl ether, etc., epoxy compounds containing amino groups in the molecule, polyvalent amine compounds, polyaziridine compounds, or polyvalent isocyanate compounds are comparatively irritating to skin, producing worries about safety when unreacted crosslinking agent remains if they are used as a crosslinking agent of the water absorbent resin for hygienic materials which are likely to come in contact with the skin of babies and infants.
On the other hand, when reactants of epihalohydrin with ammonia or low-molecular primary amine such as ethylenediamine are used for the crosslinking agent, the reactant is a compound in which the amine is simply added to the epoxy group of epihalohydrin, and because no active functional group exists in the molecule of this compound, no effective crosslinking takes place even if heated and the improvement of water absorbency under loading is insufficient.